R. O. Clements and E. J. AsterakiInstitute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon EX20 2SB, UK.
Introduction
Development of novel system
Choice of cereal
Pests, diseases and weeds
Acknowledgement
References
Temperate legumes, grown in association with grasses, can fix over 250 kg of N ha -1 annually (Halliday and Pate, 1976). On average, about 100kg ha -1 is harvested directly as legume herbage, 50 kg ha -1 indirectly as extra grass (Cowling, 1982) and the remaining 100 kg accumulates as organic N in the soil.
Usually the realization of this organic N is dependent on ploughing and arable cropping; its value is reflected by the reduced requirements of the subsequent crops for fertilizer N (Clement and Williams, 1967).
The development of drills capable of introducing seed into undisturbed soil made it possible to sow a non-leguminous species in an established legume sward. In such a system the non-legume might benefit form both residual and current nitrogen fixation. Survival of the legume understorey would lead to the process being repeated with a succession of crops.
A system is being developed at IGER (Jones, 1992) to grow cereals, especially winter wheat, in a permanent understorey of white clover. The system is straightforward. At first a pure sward of white clover is established, given a year's growth to build up fertility and then grazed hard during the autumn. The cereal crop is then direct-drilled into the clover. A standard Hunter Rotaseeder is particularly suited for this purpose. The cereal is grown with minimal or now nitrogen fertiliser and harvested conventionally. After harvest the clover is allowed to recover, grazed down hard and re-drilled with cereal in the autumn or spring to repeat the cycle. The continuous presence of a crop should minimise the loss of nitrogen through leaching because the roots of cereal and clover absorb any mineralised nitrogen. The presence of leaf area for the full period of growth maximises the use of solar energy.
Establishing cereals successfully and maintaining the desired balance of components to ensure reasonable cereal yield and clover survival is a skilful operation (Table 1). Cereal establishment was assured by suppressing the clover through grazing heavily with sheep.
Clover survival was aided by choosing cereal varieties with short, stiff straw and drilling in wider than normal rows. This reduced the risk of lodging and also allowed some light to penetrate the cereal canopy. Clover recovery was immediate from existing growing points.
Table 1. Yield of grain (t/ha, 15% moisture) of barley and oats grown with and without N, either conventionally or in a clover understorey.
|
Treatment |
Barley |
Oats |
||
|
Zero N+ 100kg N/ha |
Zero N+ 100k N/ha |
|||
|
Ploughed/conventional seedbed |
5.96 |
5.19* |
4.80 |
3.88* |
|
Clover understorey |
5.66 |
5.31 |
5.88 |
3.55* |
|
* crop lodged |
|
|
|
|
Trials with a range of cereals showed that winter wheat and spring barley appeared to be the most compatible for maintaining the clover sward. However spring barley may require agrochemical suppression of the clover for good establishment.
Winter barley tillered profusely and became too competitive, even at modest seed rates and wide drills. Spring wheat and spring oats produced few tillers allowing the clover to become dominant. Winter oats are too tall and intercept nearly all the light for long periods, leading to the clover dying. Winter wheat, since it tillers less, is best suited to the system. The density of the wheat canopy can be manipulated more easily than other cereals by altering seed rates. For example L. Jones and R.O. Clements (in press) sowed winter wheat at c. 100, 200 and 300 seeds/m2 and found that a seed rate of 200/m2 i.e. somewhat less than that used conventionally, gave the optimum cereal crop density. In their work, done on a soil with a particularly low inherent N status, there was also a need to increase fertilizer inputs to 100 kg N/ha to maintain acceptable grain yields.
Pest and disease problems to date using the clover understorey system have been much reduced, although the reasons for this are not clear. Populations of aphids were low or below the level of detection in plots where the clover was present and did not need control with insecticides. By contrast an adjacent area, which was grown conventionally, received several sprays.
Although there is no objective evidence, we believe that the dense year-round cover of vegetation provided by the clover offers a refuge for many species of predatory invertebrates. The most important of these are probably spiders, carabid and staphylinid beetles which feed on cereal aphids. Also it is known that alighting cereal aphids tend to select particular patterns of vegetation. The continuous ground cover achieved with the system could reduce the number of aphids colonising the cereal crop.
Cereal diseases have not been a problem. This may be partly because we used seed dressed with fungicides and we selected disease resistant varieties. Also, the wide row spacing and the more open crop canopy probably altered the microclimate to make it less suitable for disease development.
So far, weeds have not been a problem. But they are likely to be more troublesome than in a conventional crop because only a limited number of herbicides are available to control broad-leaved weeds without damaging the clover. Prevention is better than cure and keeping a good canopy to prevent weed invasion seems to be the best solution.
This work was done as part of that for a commission by the Ministry of Agriculture, Fisheries and Food.
CLEMENT, C.R, and WILLIAMS, T.E. (1967) Leys and organic matter. II The accumulation of nitrogen in soils under different leys. Journal of Agricultural Science. Cambridge. 69, 133-138.
COWLING, D.W. (1982) Biological nitrogen fixation and grassland production in the UK. Transactions of the Royal Society. London. 296, 397-404.
HALLIDAY, J. and PATE, J.D. (1976) The reduction assay as a means of studying nitrogen fixation in white clover. Journal of the British Grassland Society, 31, 29-35.
JONES, L. (1992) Preliminary trials using a white clover (Trifolium repens L.) understorey to supply the nitrogen requirements of a cereal crop. Grass and Forage Science. 47, 366-374.
JONES, L. and CLEMENTS, R.O. (in press). Development of a low-input system for growing wheat in a permanent understorey of white clover. Annals of Applied Biology, 123,
